Approximately 8% of the genomes of mammals, including humans and mice, are comprised of retroviral elements acquired by infection of germ line cells during the course of evolution. Retroviral insertions in our genome number about 40,000. Most endogenous retrovirus elements are defective for replication however several contain one or more viral genes that are expressed during development and certain physiological or pathological conditions. Little is known about the control of retrovirus expression or the influence of such expression on the physiology or pathology of the host. An extensively investigated group of endogenous retroviruses are those giving rise to recombinant murine leukemia viruses (MuLVs) in mice. Upon infection of mice with exogenous ecotropic MuLVs, members of this group undergo recombination to generate new MuLVs with an altered infectious host range. Recombination requires transcription of the endogenous retroviruses. Although the endogenous polytropic proviruses are transcribed; replication of the endogenous polytropic viruses in the absence of recombination has not been observed. This may, in many cases, reflect defects such as point mutations or deletions in the endogenous viral genome but may also be influenced by the activity of various restriction factors. The fact that exogenous MuLVs are capable of replicating in mice indicates that they have evolved mechanisms to circumvent the activity of at least some of the restriction factors such as the murine APOBEC3. Thus, exogenous retroviruses might facilitate through complementation, active replication of endogenous retroviruses. We have found that infection of mice by an exogenous virus results in the infectious transfer of complete endogenous proviral genetic sequences. This includes proviruses which are severely defective and possess large deletions as well as proviruses that are full-length. Furthermore, the transferred sequences are transcribed and packaged into virions released from the newly infected cells. At early times after infection with the Friend MuLV, packaging and transfer of intact endogenous retroviruses is much more prevalent than recombination. Endogenous retroviruses are transferred as early as one day after infection. Thus, the transcripts are captured within a single in vivo replication cycle and originate from among the initially infected host cells. The mobilization of intact endogenous retroviruses is unprecedented and may have important implications for the involvement of endogenous retroviruses in disease processes. In 2012 we have extended our observations to further characterize the endogenous viruses mobilized after infection by exogenous retroviruses. We have found that the mobilized endogenous viruses appear to be specifically limited to endogenous polytropic proviruses at the exclusion of other endogenous retroviral elements. The endogenous polytropic proviruses are comprised of two structural subclasses termed Polytropic (PT) and Modified Polytropic (mPT). We observed a distinct shift in the subclass of proviruses from the mPT subclass to the PT subclass of polytropic proviruses detected during the course of infection. These results suggest the spread of infection to cells expressing different classes of polytropic viruses or an alteration in the expression of the endogenous proviruses in infected cells. Exogenous mouse retrovirusesencode a glycosylated gag protein (gGag) originating from an alternate translation start site upstream of the methionine start site of the gag structural polyproteins. The functions of gGag remain unclear, but mutations that eliminate its synthesis severely impede in vivo replication of the virus with little effect on replication in fibroblastic cell lines. APOBEC3 proteins have evolved as innate defenses against retroviral infections. Both mice and humans express APOBEC3 proteins that have cytidine deaminase activity leading to hypermutation of viral transcripts and inactivation of infecting retroviruses. HIV encodes the VIF protein to evade human APOBEC3G (hA3G), however mouse retroviruses do not encode a VIF homologue and it has not been understood how they evade mouse APOBEC3 (mA3). We have found that a mouse retrovirus utilizes its glycosylated gag protein (gGag) to evade APOBEC3. gGag is critical for infection of in vitro cell lines in the presence of APOBEC3. Furthermore, a gGag-deficient virus restricted for replication in wild-type mice replicates efficiently in APOBEC3 knockout mice implicating a novel role of gGag in circumventing the action of APOBEC3 in vivo. In 2012 we have continued to focus on the elucidation of the mechanism by which the gGag protein abrogates the action of APOBEC3. Human APOBEC3G (hA3g), is counteracted by the Vif protein of HIV which depletes hA3g from infected cells by facilitating its degradation through the proteosome. We have found that a gGag-containing MuLV does not deplete mA3 from an infected cell nor is the mutation rate altered in gGag-negative viruses We have observed that Inhibition by mA3 packaged within gGag-deficient virions correlates with a decrease in the level of transcripts upon infection of cells and appears largely independent of deamination activity. The endogenous retroviral envelope glycoprotein, gp70 is implicated in murine lupus nephritis. This protein is secreted by hepatocytes as an acute phase protein and has been believed to be a product of an endogenous xenotropic virus. However, since endogenous polytropic viruses encode gp70s that are closely related to xenotropic gp70, these viruses could be additional sources of serum gp70. To better understand the genetic basis of the expression of serum gp70, we analyzed the abundance of xenotropic and polytropic gp70 RNAs in livers and the genomic composition of corresponding endogenous proviruses in various strains of mice, including two different Sgp (serum gp70 production) congenic mice (Sgp3 and Sgp4). These studies revealed a significant contribution of polytropic gp70s to serum gp70. These studies were extended to show that expression levels of the mPT subclass of polytropic MuLVs, are highly elevated in mice which develop systemic lupus erythematosus and are under the control of the Sgp3 locus. Further, we have found that Sgp3 and Sgp4 independently regulate the transcription of distinct and restricted sets of xenotropic viral sequences in trans, thereby promoting the production of nephritogenic gp70 autoantigens. Among these endogenous viruses induced by Sgp3 and Sgp4 are two potentially replication competent xenotropic viruses. In 2012 these studies were extended to examine in more detail the effects of Sgp3 on the transcription of mPT viruses. Our results indicate that Sgp3, by itself, regulated the transcription of only 3 of 13 endogenous mPT viruses. This increase appeared to be the result of co-regulation of genes in which the proviruses were integrated in the same transcriptional direction. However, upon stimulation of TLR7, Sgp3 congenic mice expressed additional polytropic viruses, one of which is highly up-regulated, contains intact gene sequences and is potentially replication competent. The induction of a replication-competent mPT could potentially result in spread of the virus and facilitate the spread of xenotropic MuLVs in the host through pseudotyping. Such a spread could contribute to the development of autoimmune responses against serum gp70 through the activation of TLR7.